efi_loader: provide new doc/README.uefi

Provides information about

- usage of the bootefi command
- overview of UEFI
- interaction between U-Boot and EFI drivers

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Acked-by: Leif Lindholm <leif.lindholm@linaro.org>
Signed-off-by: Alexander Graf <agraf@suse.de>